
theorem
  for L1,L2 be non empty RelStr st L1,L2 are_isomorphic holds L2,L1
  are_isomorphic
proof
  let L1,L2 be non empty RelStr;
  given f being Function of L1,L2 such that
A1: f is isomorphic;
  consider g being Function of L2,L1 such that
A2: g = (f qua Function)" and
A3: g is monotone by A1,WAYBEL_0:def 38;
  f = (g qua Function)" by A1,A2,FUNCT_1:43;
  then g is isomorphic by A1,A2,A3,WAYBEL_0:def 38;
  hence thesis;
end;
